Color removal from the effluent streams of paper mills continues to be a problem within the pulp and paper industry. It is necessary that these downstream wastewaters be treated for color removal prior to discharge into public waterways.
The United States wood pulp production capacity is approximately 60 million tons per year. Since the average cellulose content of wood is about 40%, 150 million tons of wood are needed to produce this 60 million tons of pulp. The difference between these two numbers represents the lignin and hemicellulose which must be removed or separated in the pulping process in order to free the cellulose fibers.
The pulping process, however, does not remove 100% of the lignin present in the wood, with approximately 5% remaining after either kraft or sulfite pulping (for mechanical pulping the amount is considerably higher). If a high grade paper is the desired end product, this 5% residual lignin must be removed, and is accomplished by bleaching the pulp.
Since over 35% of the pulp produced in the United States is bleached, there are about one million tons of lignin removed each year at the bleach plant, and most of this in the caustic extraction stage. This number is significant because in the removal process (i.e., bleaching), most of this residual lignin is solubilized. This solubilized lignin is a strong absorber of visible radiation resulting from the conjugation of unsaturated and quinoidal moieties formed during the oxidation step in the bleach plant. Consequently, the bleach plant effluent is highly colored. Although there are other sources of color in paper mill waste effluent, it is readily apparent that where bleaching is performed its effluent can be expected to be the major contributor of waste color. Indeed, at kraft, bleach mills the effluent from the first caustic extraction stage accounts for at least 70% of the waste color.
The goal of the pulping and bleaching operations is the removal of lignin and hemicellulose from the cellulose fiber in the wood. The 95% that is removed by pulping is often burned as fuel in the process of recovering the inorganic chemicals present in the black liquor. In the bleaching operation, the 5% residual lignin is separated from the fibers by degradation and solubilization and ends up in the wastewater. Chemical removal can, therefore, only be accomplished by reducing this solubility, which has proved to be a difficult task.
The process of color removal from the effluent stream is further complicated by the presence of lime, solid particulate matter like pulp, clay, dispersants/surface active materials and polymers used during various stages in the papermaking process. The solid particulate matter is commonly referred to as anionic trash.
Most governmental regulations pertaining to color removal from the effluent stream of a papermaking process are directed to true color, i.e., defined by the EPA/NCASI test as the absorbance of 465 mm of light by a sample adjusted to a pH of 7.6 and filtered through a 0.8 micrometer filter paper. Color is reported in standard color units (scu) which represents the concentration of a color standard solution producing an equivalent degree of absorbance (1 scu=1 mg/1 platinum as chloroplatinate). Nevertheless, there is increasing pressure on pulp and paper mills to lower the apparent color of the effluent water because that is the color visible to the naked eye as the effluent flows into public waterways. Apparent color is unfiltered and not pH adjusted, and results in part from particles that scatter light. There are occasions when the true color of a system that has undergone treatment is low, but the corresponding apparent color is high. This problem is commonly caused by the presence of suspended particulate matter that causes an increase in the turbidity of the system. Therefore, it is important that any new treatment for color removal should not only remove the true color of the effluent, but should also lower the apparent color as well.
The pressure to remove color comes primarily from state environmental agencies. Previously, it was thought that the discharge of colored waste affected only the aesthetic value of the receiving body of water; however, biologists are becoming increasingly concerned about possible toxic effects, the effect of reduced light transmittance through the water causing reduced levels of photosynthetic activity, and of course, the resultant drop in dissolved oxygen concentration because of this drop in activity. Furthermore, although these colored, waste products are fairly refractory towards biological oxidation and since they become degraded in the aquatic environment, the oxidation products may be potentially harmful.
It has been shown that by-products are water soluble, and that a significant amount is produced. This puts severe demands on chemicals to be used for color removal. There are techniques already available, however, that can remove greater than 90% of the color from either total mill effluent or isolated waste streams, such as from the caustic extraction stage of the bleach plant. These techniques include chemical (e.g., alum, ferric, lime or polyelectrolytes), biological (e.g., white rot fungus) and physical processes (e.g., ultrafiltration, ion exchange and carbon absorption). None enjoys widespread use because of unfavorable economics.
The demands on a product used in a color removal application are quite severe, i.e., the product must be capable of reacting with the color bodies in a manner which results in their becoming insoluble and, because of the extremely large amount of color bodies produced, the color removal product must work at very low weight ratios relative to the organics being removed or its use will be precluded by prohibitive costs.
A common problem associated with conventional chemical treatment methods, such as polymer made from epichlorohydrin/dimethylamine (Epi/DMA), is the fact that those polymers cannot lower the color of a system below a certain value beyond which they tend to re-disperse the color. This problem is commonly referred to as "overdosage."
In virtually all biological (i.e., non-enzymatic) color removal processes, white rot and other types of fungi are used. Although these fungi are very effective in removing color, they require acidic pH (about 5 or lower), an additional carbon source (e.g., glucose, sucrose, xylose, etc.), and/or attachment to an inert support. Maintaining the pH of the wastewater at about 5 or lower and the addition of a carbon source is expensive in terms of both additional chemicals and process equipment.
Some examples of biological color removal processes are set forth in U.S. Pat. Nos. 4,199,444 (Blair et al.), which issued on Apr. 22, 1980, and 4,266,035 (Blair et al.), which issued on May 5, 1981. Both patents relate to a process of decolorizing pulp and paper mill wastewater which involves the treatment of the wastewater effluent with a microbial strain of Pseudomonas aeruginosa under aerobic conditions. This particular microorganism requires the utilization of carbon-containing compounds for growth, e.g., glucose. The mutant strain Pseudomonas aeruginosa 4-5-14 can be employed alone or in combination with other microorganisms conventionally used in microbiological treatment of wastes. It also disclosed the use of any variants of Pseudomonas aeruginosa 4-5-14 alone or in combination.
Still others have used enzymes to remove color from pulp and paper wastewaters. The Ferrer et al. article, entitled "Decolorization of Kraft Effluent by Free and Immobilized Lignin Peroxidases and Horseradish Peroxidase", Biotechnology Letters, Vol. 13, No. 8, pp. 577-582 (1991), demonstrated that immobilized lignin peroxidase, horseradish peroxidase or lyophilized fungal cultures have a considerable potential for treatment of kraft effluent.
The article by Davis and Burns, "Decolorization of Phenolic Effluents by Soluble and Immobilized Phenol Oxidases", Applied Microbiology and Biotechnology, 1990, 32:721-726, discloses that horseradish peroxidase removed color from pulp mill, cotton mill hydroxide and cotton mill sulphide effluents, but rapid and irreversible enzyme inactivation took place. Peroxidase oxidize phenolics to aryloxy radicals, which spontaneously polymerize to form insoluble complexes; these can be removed by precipitation, filtration or centrifugation. Moreover, Davis and Burns disclosed that hydrogen peroxide concentration had a marked effect on the decolorizing ability of entrapped horseradish peroxidase (HRP) due to inactivation of the released enzyme. That is, the hydrogen peroxide acted to compensate for the HRP inactivation.
The present inventor has discovered through extensive observations that the peroxidase/H.sub.2 O.sub.2 treatment system discussed above in the Davis and Burns article does not take into consideration the feedback inhibition on the enzyme during treatment of pulp mill wastewater effluents. This feedback inhibition on the enzyme that occurs during treatment prevents the color removal process from being carried out for a period of time sufficient to remove regulated amounts of color from the effluent.
In this regards, the present inventor has developed a two step treatment program that utilizes enzymes to oxidize the color forming organic compounds in the wastewater effluent and microorganisms to degrade the products of the enzymatic oxidation and, consequently, remove the feedback inhibition and re-activate the enzyme so that color removal can be carried out much further.
None of the conventional biological color removal processes known to the present inventor involve a two step process of treating a pulp or paper mill wastewater effluent with a phenol oxidase enzyme followed by a tannin-degrading microorganism.
The present invention also provides many additional advantages which shall become apparent as described below.